diff --git a/file.go b/file.go
index 3432ddf..6e12532 100644
--- a/file.go
+++ b/file.go
@@ -45,8 +45,11 @@ type file struct {
 
 func (f *file) export(dstDir string) error {
 	dstPath := filepath.Join(dstDir, f.path)
-	if len(f.asset) > 0 && fileCached(f.asset, dstPath) {
-		return nil
+	if len(f.asset) > 0 {
+		dstInfo, err := os.Stat(dstPath)
+		if err == nil && dstInfo.ModTime().Unix() >= f.ModTime().Unix() {
+			return nil
+		}
 	}
 
 	if err := os.MkdirAll(path.Dir(dstPath), 0755); err != nil {
diff --git a/goldsmith.go b/goldsmith.go
index cce2b5e..fa32316 100644
--- a/goldsmith.go
+++ b/goldsmith.go
@@ -24,6 +24,7 @@ package goldsmith
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"os"
 	"time"
@@ -73,6 +74,10 @@ func NewFileFromAsset(path, asset string) (File, error) {
 		return nil, err
 	}
 
+	if info.IsDir() {
+		return nil, errors.New("assets must be files")
+	}
+
 	f := &file{
 		path:    path,
 		Meta:    make(map[string]interface{}),
diff --git a/util.go b/util.go
index 6897388..f5d0164 100644
--- a/util.go
+++ b/util.go
@@ -55,17 +55,3 @@ func scanDir(root string, infos chan fileInfo) {
 		return nil
 	})
 }
-
-func fileCached(srcPath, dstPath string) bool {
-	srcStat, err := os.Stat(srcPath)
-	if err != nil {
-		return false
-	}
-
-	dstStat, err := os.Stat(dstPath)
-	if err != nil {
-		return false
-	}
-
-	return dstStat.ModTime().Unix() >= srcStat.ModTime().Unix()
-}